Abstract: We validate a $R_p=2.32\pm 0.24R_\oplus$ planet on a close-in orbit
($P=2.260455\pm 0.000041$ days) around K2-28 (EPIC 206318379), a metal-rich
M4-type dwarf in the Campaign 3 field of the K2 mission. Our follow-up
observations included multi-band transit observations from the optical to the
near infrared, low-resolution spectroscopy, and high-resolution adaptive-optics
(AO) imaging. We perform a global fit to all the observed transits using a
Gaussian process-based method and show that the transit depths in all passbands
adopted for the ground-based transit follow-ups ($r'_2, z_\mathrm{s,2}, J, H,
K_\mathrm{s}$) are within $\sim 2σ$ of the K2 value. Based on a model of
the background stellar population and the absence of nearby sources in our AO
imaging, we estimate the probability that a background eclipsing binary could
cause a false positive to be $< 2\times 10^{-5}$. We also show that K2-28
cannot have a physically associated companion of stellar type later than M4,
based on the measurement of almost identical transit depths in multiple
passbands. There is a low probability for a M4 dwarf companion ($\approx
0.072_{-0.04}^{+0.02}$), but even if this were the case, the size of K2-28b
falls within the planetary regime. K2-28b has the same radius (within
$1σ$) and experiences a similar irradiation from its host star as the
well-studied GJ~1214b. Given the relative brightness of K2-28 in the near
infrared ($m_\mathrm{Kep}=14.85$ mag and $m_H=11.03$ mag) and relatively deep
transit ($0.6-0.7\%$), a comparison between the atmospheric properties of these
two planets with future observations would be especially interesting.